1. Field of the Invention
The present invention relates to a rubber-modified phenolic resin composition which can be used as a main resinous component or a curing agent for a curable resin, and a method of manufacturing the same. More particularly, it relates to a rubber-modified phenolic resin composition which can be used to form a cured product having good impact resistance and thermal shock resistance when the composition is cured and which can give good impact resistance and thermal shock resistance to a cured product when the composition is used as a curing agent for other curable resins. The present invention also relates to an encapsulant for electronic or electric components, having low stress property and good moisture resistance.
2. Description of the Prior Art
When a phenolic resin is cured, it can form a cured product which is lustrous, which is not easily fused, which has good weathering resistance, water resistance, mechanical strength, and thermal and electrical insulation properties, and which is not easily affected by chemicals. For this reason, the phenolic resin is widely used as varnish, a laminated sheet material, a molding material, and an insulating material in various industrial fields such as fields of electrics and electronics.
In addition, a phenolic resin, especially a novolak-type phenolic resin is also used as a curing agent for other curable resins such as an epoxy resin and a bismaleimide resin. A composition mainly consisting of an epoxy resin is widely used as a material for sealing electronic components such as diodes, transistors, and ICs. Examples of a curing agent for an epoxy resin other than the novolak-type phenolic resin are acid anhydrides and aromatic amines. However, an epoxy resin composition using the novolak-type phenolic resin as a curing agent has good molding properties and moisture resistance, has no toxicity, and is inexpensive as compared with the one using other curing agents. Therefore, the epoxy resin composition using the novolak-type phenolic resin as a curing agent is widely used as an encapsulant for electronic devices such as semiconductor devices.
However, a cured phenolic resin is brittle and has a poor impact resistance although it has the above superior properties. When such a phenolic resin is used as a curing agent for an epoxy resin or a polyimide resin, these drawbacks also appear.
In addition, the epoxy resin composition using the novolak-type phenolic resin as a curing agent shrinks when it is molded and cured, thereby degrading the reliability of semiconductor devices sealed therein. That is, when a semiconductor device sealed in such a resin composition is subjected to a thermal cycle test, open of a bonding wire, resin cracks, pellet cracks, and the like occur. For this reason, such semiconductor devices cannot function as electronic components.
Therefore, it is conventionally proposed to add tung oil or polybutadiene having a functional group to the novolak-type phenolic resin to modify it. However, a cured product obtained by curing a modified novolak-type phenolic resin prepared in such manner has poor electric properties, the surface of the cured product is contaminated, and its strength is degraded. Furthermore, the cured product obtained from abovementioned novolak-type phenolic resin does not have sufficiently good impact resistance, thermal shock resistance, and low stress property.